The present invention relates in general to telecommunication techniques. More particularly, the invention provides a system and method for improving the accuracy of demodulating received data signals. In certain specific embodiments, statistical tools have been implemented for detecting possible errors of data transmission. Merely by way of example, the invention is described as it applies to communication networks, but it should be recognized that the invention has a broader range of applicability.
Telecommunication techniques existed almost as long as human history. Before electrical means of information exchange were possible, people devised various techniques for transmit information over distances. Visual and/or audio relay techniques for transmitting information over long distances have been used. For example, thousands of years ago border patrols in China used smoke signals to warn the central government of foreign invasions. Native Americans have also been known to use smoke signals.
With the invention of telegraph in the early nineteenth century, various electrical means of transmitting information have been developed. For example, various type of technologies (e.g., telephone, facsimile, radio, etc.) have been developed.
In a transmission process, the information that is typically modulated at the transmitting end, demodulated at the receiving end, and transmitted through a medium. For example, digital transmission of data typically involves the following steps: (1) encoding data, (2) modulating data, (3) transmitting data over a medium, (4) demodulating data, and (5) decoding data. Typically, during the transmission over a medium (e.g., wire, air, etc.), modulated data suffer from both noise and attenuation loss. Therefore it is often necessary to determine whether the transmitted data contain any errors.
One of a commonly used methods for detecting error is hybrid automatic request (HARQ) method, which offers good performance for many types of network work and particularly wireless networks. The application of HARQ method is described below.
At the transmitting end, a data frame is encoded in a code word, and a code word is divided into several segments. Each code word segment includes several code word symbols. The transmitting end selects one or multiple code word segments in each transmission, and a concrete number of segments and code word symbols are constrained by the resource. These code word symbols are modulated to generate several modulation symbols, and then these modulation symbols are sent out.
The receiving end decodes the currently received and/or previously received code word segments. If the decoding is successful, then it feeds back a transmission successful message to the transmitting end, and transmission of data frame is complete. If decoding fails, then it feeds back a failure message to the transmitting end. The transmitting end then carries out an HARQ for retransmission of this data frame for the second attempt. Typically, the code word segments chosen for retransmission might be the same or different compared with the previous transmission/transmissions. Depending upon application, mapping of the encoding symbols to modulation symbols also might be changed, and the modulation order might be changed as well. For example, in HARQ transmission of a “Modulation Order Step-Down” mode, the code word symbols for HARQ transmission might use Quadrature amplitude modulation (QAM) (e.g., 16QAM) to modulate for the first time, and the code word symbol for HARQ transmission might use Phase-shift keying (e.g., 8PSK) to modulate for the second time.
FIG. 1 is a simplified diagram illustrating operation of an HARQ system. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 1, the (b0, b1, . . . , bm−1) row represents coding symbols, the (S0, S1, . . . , Sn−1) row represents modulation symbols, and the connecting line between the coding symbols and the modulation symbols represents certain coding symbols that were used for generating of certain modulation symbols. The coding symbols have more than one connecting lines, which represents this coding symbol has been repeatedly transmitted during HARQ transmission.
To further explain the operation of an HARQ system, the following expressions are presented below. The mapping procedure from coding symbols to modulation symbols is one-to-one mapping procedure as the followings:φ: {right arrow over (b)}=(b0, b1, . . . , bm−1)→{right arrow over (s)}=(s0, s1, . . . , sn−1)  (Equation 1)φ−1: {right arrow over (s)}=(s0, s1, . . . , sn−1)→{right arrow over (b)}=(b0, b1, . . . , bm−1)  (Equation 2)
Where, {right arrow over (b)}=(b0, b1, . . . , bm−1) represents the vector of the coding symbols, {right arrow over (r)}=(r0, r1, . . . , rn−1) represents the vector of the received symbols, and {right arrow over (s)}=(s0, s1, . . . , sn−1) represents the vector of the transmitted modulation symbols.
As an example, Equation 1 describes the operation of modulating coding symbols, and Equation 2 describes the operation of demodulating received signals. It is desired that the coding symbols after demodulation are the same as before modulation (i.e., data being faithfully transmitted). To ensure that the demodulated received signals are accurate, various conventional techniques have been developed. For example, log likelihood ratio (LLR) value is used to determine the likelihood of the received signals being accurate. As an example, upon determining the LLR value for a demodulated signals is lower than a threshold LLR value, the receiver of the signals send a request for resending data. Unfortunately, conventional techniques are often inadequate.
Therefore, an improved method and system for demodulation is desired.